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S.J. Coles et al. / Journal of Organometallic Chemistry 586 (1999) 234–240
complexes (L†, L††=various P donors), rather than
the reaction of tertiary phosphine mixtures with
PdCl2(PhCN)2. Thus, within the two pairs of satu-
rated/unsaturated phosphines used in the present
study rates of PhCN displacement by these phosphi-
nes will be similar. Therefore, the isolation of cis and
trans isomers of both symmetrically and unsymmetri-
cally substituted phosphine complexes is not surpris-
ing.
Table 2
31P{1H}-NMR data for the reaction of 1 with one and four equiva-
lents of thf·BH3
Products
1+1thf·BH3 31P{1H} 1+4thf·BH3 31P{1H}
l (ppm)
l (ppm)
LI a
26.0
21.7
LV a
LI a
25.9
21.0
LV a
Sym-[PdCl2LI]2
Sym-[PdCl2LV]2
Cis-[PdCl2LI2]
Absent
33.8
Cis-[PdCl2LV2 ]
Absent
29.8 b
30.7
2.2. Reacti6ity with BH3 and 9-BBN
Cis-[PdCl2LILV]
Trans-[PdCl2LI2]
Trans-[PdCl2LV2 ]
20.8 b
13.6
20.8 b
13.6
29.7 b
The reactivity of BH3 with vinyl- and allyl-substi-
tuted phosphines has been investigated by Imamoto
[2] and Schmidbaur [3]. Both ligands readily form
phosphine–borane adducts with thf·BH3 in preference
to hydroboration of the alkene function. Moreover,
internal hydroboration and cyclisation does not pro-
ceed even under forcing conditions, although addi-
tional free thf·BH3 does result in alkene
hydroboration. Thus, addition of thf·BH3 to CH2Cl2
and CHCl3 solutions of 1 and 2 offers the potential of
metal reduction, phosphine–borane formation, or
alkene hydroboration. In dilute (millimolar) solutions
of 1 the addition of thf·BH3 produces a series of
31P{1H}-NMR resonances, Table 2. Using one equiva-
lent of thf·BH3 per mole of 1 phosphine abstraction
from 1 to give phosphine–borane is the predominant
reaction; the ligand-deficient palladium fragments
dimerising to 3. There is some evidence for limited
BH3 reactivity with the alkene groups of coordinated
phosphines as evidenced by the assignment of
31P{1H}-NMR resonances to PdCl2LILV (LI=
PPh2CHꢀCH2; LV=PPh2CH2CH2BH2). In view of the
recognised lability of the general class of PdX2P2 com-
plexes, it is perhaps surprising that redistribution of LI
and LV does not occur to afford a mixture of
PdCl2LI2, PdCl2LILV and PdCl2LV2 based on statistical
and thermodynamic factors. However, such exchanges
may not be significant for PdCl2LILV species, since
similar mixed phosphine systems appear relatively in-
ert [6].
Absent
20.9 c
19.2
Trans-[PdCl2LILV] 12.2 c
12.1 b
18.9 d
20.8 b
LIBH3, LVBH3
19.0 d
a LI, PPh2CHꢀCH2; LV, PPh2CH2CH2BH2.
b 2JPP=0 Hz.
c 2JPP=550 Hz.
d Broad.
ligand abstraction to form phosphine–borane is the
principal form of reactivity with alkene hydroboration
being a secondary feature, although in this instance
[PdCl2P]2 species could not be observed by 31P{1H}-
NMR (Table 3). 1:4 Complex:thf·BH3 stoichiometries
result in significantly more phosphine abstract ion and
the clear evidence for the formation of [PdCl2P]2 spe-
cies. Alkene hydroboration is more extensive with
complexes containing both one and two hydroborated
alkenes being observed. When compared with the re-
activity of the vinyl phosphine complex, 1, the hy-
droboration of alkene functionalities in 2 is less
regioselective. For the former the a-PPh2 group
strongly influences BH3 addition at the alkene func-
tion resulting only in PPh2CH2CH2BH2 (LV) forma-
tion. As would be expected, this PPh2 influence is less
significant for the allyl substituted ligand. Although
terminal BH2 addition to afford PPh2(CH2)3BH2 (LVI)
still occurs most readily, there is also evidence for the
presence of species containing PPh2CH2CH(BH2)CH3
(LVII). Identification of these final species must be
considered tentative, since assignments are made by
analogy with the saturated/unsaturated mixed ligand
systems described in Section 2.2.
On reacting four equivalents of thf·BH3 with 1
phosphine abstraction to form phosphine–borane con-
tinues be a significant feature of reactivity. However,
alkene hydroboration becomes more extensive with cis
and trans PdCl2LILV and PdCl2LV2 being observed.
Clearly product distribution is dependant on stoi-
chiometry. There is also evidence for a concentration
dependance; the use of similar reaction stoichiometries
in neat thf·BH3 or the addition of thf·BH3 to 10−1M
CH2Cl2 solutions results in the reduction of 1 to
metallic palladium and the isolation of phosphine–bo-
rane.
Stoichiometric reactions of 9-BBN and 2 proceed in
a similar manner to those of thf·BH3. 31P{1H}-NMR
spectroscopy indicates the presence of unreacted 2, the
dimeric complex 4, a phosphine–borane adduct (9.8
ppm) and the cyclised phosphine–borane adduct 6
(15.4 ppm) (Scheme 1). The isolation of a single palla-
dium complex containing a hydroborated phosphine
again appears to be precluded by the establishment of
equilibria and by the potential for palladium reduction
The reactivity of 2 with thf·BH3 is broadly similar
to that of 1; at 1:1 ratios of complex and thf·BH3